This invention relates to chemical mechanical polishing, and more particularly to a carrier head for chemical mechanical polishing.
Integrated circuits are typically formed on substrates, particularly silicon wafers, by the sequential deposition of conductive, semiconductive or insulative layers. After each layer is deposited, it is etched to create circuitry features. As a series of layers are sequentially deposited and etched, the outer or uppermost surface of the substrate, i.e., the exposed surface of the substrate, becomes increasingly nonplanar. This nonplanar surface can present problems in the photolithographic steps of the integrated circuit fabrication process. Therefore, there is a need to periodically planarize the substrate surface. In addition, planarization is needed when polishing back a filler layer, e.g., when filling trenches in a dielectric layer with metal.
Chemical mechanical polishing (CMP) is one accepted method of planarization. This planarization method typically requires that the substrate be mounted on a carrier or polishing head. The exposed surface of the substrate is placed against a polishing pad, such as circular pad or linear belt, that moves relative to the substrate. The polishing pad may be either a “standard” or a fixed-abrasive pad. A standard polishing pad has a durable roughened or soft surface, whereas a fixed-abrasive pad has abrasive particles held in a containment media. The carrier head provides a controllable load on the substrate to push it against the polishing pad. Some carrier heads include a flexible membrane that provides a mounting surface for the substrate, and a retaining ring to hold the substrate beneath the mounting surface. Pressurization or evacuation of a chamber behind the flexible membrane controls the load on the substrate. A polishing slurry, including at least one chemically-active agent, and abrasive particles if a standard pad is used, is supplied to the surface of the polishing pad.
The effectiveness of a CMP process may be measured by its polishing rate, and by the resulting finish (absence of small-scale roughness) and flatness (absence of large-scale topography) of the substrate surface. The polishing rate, finish and flatness are determined by the pad and slurry combination, the relative speed between the substrate and pad, and the force pressing the substrate against the pad.
A reoccurring problem in CMP is non-uniform polishing, i.e., variation in the polishing rate across the substrate surface, resulting in non-uniform substrate thickness. One cause of non-uniform polishing is substrate deformation, e.g., bowing of the substrate.
Another problem with CMP is that it is a “dirty” process. Specifically, foreign material is introduced while the polishing process is performed. However, this foreign material needs to be removed before the substrate is further processed to prevent substrate contamination. Therefore, in the case of CMP, slurry introduced onto the substrates should be thoroughly removed at the conclusion of polishing in order to obtain a high yield of working devices on the polished substrates.